This invention relates generally to fiber optic sensors and to processing signals output from fiber optic sensors. This invention relates particularly to a multiple sensor system and simultaneous and continuous processing of all signals output from the fiber optic sensors. Still more particularly, this invention relates to a technique for separating and simultaneously monitoring and processing optical signals output from a sensor system having a multiplicity of fiber optic interferometers that provide sensor signals to a single optical fiber.
Fiber optic sensors have a wide variety of applications for sensing parameters such as temperature, pressure, strain, acoustic waves, electromagnetic waves and rotation. Strain in an optical fiber produces an optical path length change by physically changing the length of the fiber and by changing its index of refraction by means of the photoelastic effect. A number of parameters may be measured when the optical fiber is mounted in a transducer that produces a strain in the fiber when the parameter of interest changes.
The most sensitive fiber optic sensors detect changes in a parameter of interest by monitoring interference between two signals. One or both of the optical signals are exposed to the parameter. The signals have different optical paths and experience a relative phase change as the parameter changes. Commonly used interferometric sensors include Mach-Zehnder, Michelson, Fabry-Perot, ring resonator, polarimetric and two-mode fiber interferometers. Most of these sensors have two separate fibers that form sensing and reference arms. A length of a single fiber may function as an interferometer if it guides two modes that exhibit different responses to changes in the parameter being measured.
Multiplexing of fiber optic sensors is well-known. Several techniques for multiplexing fiber optic sensor are described by Kim and Shaw, Multiplexing of fiber optic sensors, Optics News, November 1989, pp. 35-42. A number of fiber optic sensors may be connected together with single distribution and return fiber optic buses sharing optical and electronic components. Signals from individual sensors must be separated so that they may be monitored without interference with signals from other sensors. Techniques for separating the signals include time division multiplexing, frequency division multiplexing, coherence multiplexing and wavelength division multiplexing.
Most existing approaches to multiplexing fiber optic sensors are based on the differences in propagation time of the optical signal from the source to the detector for different sensors. Time division multiplexing has been used when the optical carrier is amplitude modulated. In such systems each input pulse travels through a fiber to all sensors in the system. Each sensor signal has a characteristic time delay that may be used to separate the sensor signals at the detector. The return pulses of all sensors propagate along a common fiber and appear as time multiplexed signals at the output of the fiber bus. Processing these signals therefore has to be performed in a fashion which demands time division. Both the integration time and signal to noise ratio in time-division-multiplexed sensor systems decrease as the number of sensors increases.
Kim and Shaw disclose a coherence multiplexing system that includes a plurality of pathlength mismatched fiber optic interferometric sensors and a plurality of corresponding compensating interferometers all connected to the same distribution fiber. The sensing interferometers are connected in series, and the compensating interferometers are connected in parallel. The compensating interferometers have the same pathlength mismatches as the corresponding sensing interferometers.
Each sensor has unbalanced arms having path length mismatches much longer than the coherence length of the source. The mismatches in the interferometers are different so that no two possible optical paths in the sensor part of the array have the same length. No interference occurs in any of the sensors before the signal from the source goes through the compensating interferometers. The compensating interferometer for a selected sensor brings uninterfered signals from that sensor within the coherence length of the source. Signals from the sensor then interfere with signals output from the corresponding compensating interferometer. An array of N sensor interferometers may have N separate compensating interferometers and detectors or the array may have one tunable compensating interferometer.
One difficulty with the prior art coherence multiplexing system is the increase in path length mismatch required as the number of sensors increases. Another difficulty is that the compensating interferometers must be arranged so that signals from the sensing interferometers interfere with the outputs of the corresponding compensating interferometers.
Wavelength division multiplexing may be accomplished by addressing individual sensors using wavelength-selective grating reflectors or directional couplers for each sensor in the array. Signals from a number of optical sources with different wavelengths are combined using a wavelength multiplexing coupler. Sensor interferometers are connected to the distribution and return fiber buses with multiplexing couplers. Each sensor is, therefore, monitored by an optical signal with a specific wavelength for each sensor. Signals from the return bus may be separated by a multiplexing coupler or an optical grating.